α+β Type titanium alloy, a typical example of which is Ti-6Al-4V alloy, has lightness, a high strength and a high corrosion resistance, and further has weldability, superplasticity, diffusion bondability, and various other properties to be frequently used for engine parts and others in the airplane industry. α+β Type titanium alloy has an α phase of dense hexagonal crystal (hcp structure), which is a main phase, and a β phase of body centered cubic crystal (bcc structure) that coexist stably at room temperature; and turns into a single phase of the β phase in a temperature range of the β transus temperature (Tβ), or higher. Forged materials of α+β type titanium alloy are classified to materials based on α+β forging, which are each obtained by heating a starting material thereof into a temperature range lower than Tβ (α+β two-phase range) to cause the temperature of the material not to reach any temperature of Tβ and higher temperatures, and then forging the materials; and materials based on β forging, which are each obtained by heating a starting material thereof into a temperature range of Tβ (β single-phase range) and higher temperatures, and then forging the material. It is known that the two species are entirely different from each other in formed material microstructure, and accompanying the difference, the species are different from each other in material properties.
Forged titanium alloy material has an acicula α phase microstructure according to the β forging. Specifically, its microstructure is formed by as follows: a starting material thereof turns into a β single-phase in a temperature range of Tβ and higher temperatures; the β phase (β grains), which is in an equiaxial form, is crushed into a flat form by forging; and in a case where the material is subsequently cooled into a temperature range lower than Tβ and then held in this temperature range, an α phase precipitates into a membrane form along crystal grain boundaries of the β grains and subsequently another α phase precipitates into an acicula form inside the crystal grains of the β grains (the α phases are whitely represented in FIG. 2(a)). β Forging is classified into a manner of completing the forging in the β single-phase range; a manner of continuing the forging also after the temperature lowers to the outside of the β single-phase range (to the α+β two-phase range); and a manner of starting the forging after the temperature lowers to the α+β two-phase range. Furthermore, about β-forged material, the form or thickness of an α phase of crystal grain boundaries of prior β grains thereof, and the length or thickness of an acicula α phase inside the grains are varied in accordance with conditions for the forging, and conditions for cooling after the forging. Furthermore, there also exists β-forged material having no α phase in its grain boundaries. In the meantime, forged titanium alloy material has a granular α microstructure according to α+β forging (see FIG. 2(b)). In general, out of forged α+β type titanium alloy materials, forged materials obtained by β forging are better in fracture toughness than forged material obtained by α+β forging. Conversely, the forged materials obtained by α+β forging are better in fatigue strength property than the forged material obtained by β forging.
Engine parts of any airplane are required to have a high fatigue strength property and a high reliability; thus, it is inspected whether or not the parts have a defect by an ultrasonic inspection. The ultrasonic inspection is an inspection of making ultrasonic waves emitted (sent) from a probe incident to the inside of a body to be inspected from the surface thereof, and then using the same probe to receive reflected waves reflected onto a flaw and other defects therein to determine whether or not the body has an internal defect. However, an α+β type titanium alloy, in which an α phase and a β phase coexist, produces large noises caused by the material microstructure when subjected to an ultrasonic inspection whether or not the alloy is an “α+β”-forged material or β-forged material. Thus, the precision of the defect detection lowers, or a noise caused by the material microstructure is recognized as a defect by mistake, so that a problem is caused. For this reason, engine parts and others that are made of α+β type titanium alloy (hereinafter referred to as titanium alloy) have been required to be decreased in noises generated when subjected to an ultrasonic inspection, thereby being improved in ultrasonic inspectability.
Hitherto, thus, as an α+β type titanium alloy decreased in noises generated therefrom, for example, the following has been suggested: a titanium alloy rolled plate obtained by cooling a starting material thereof rapidly from the β single-phase range before the material is hot-rolled in the α+β two-phase range, thereby making the microstructure of fine, and subsequently subjecting the resultant to hot rolling and heat treatment in the α+β two-phase range, thereby yielding an equiaxial α microstructure (Patent Literature 1).